How to use Specular Buffer as a mask in a Post Process Material effect?

How would I take the Specular information of a scene and use it to mask in/out objects with a specular value of say, 1, in a post process material?
I want to apply an effect only to materials with that specular value on screen.
I figure I need to use a Post Process Material, a SceneTexture node and perhaps some maths trickery or stencils, but I’m not sure how or in what order.
Any help is appreciated!